1. Field of the Invention
The present invention relates to a field-effect transistor, and particularly, to a field-effect transistor for use in a radio-frequency region that is equal to or higher than the microwave band.
2. Description of Related Art
One of the key devices for next generation mobile communication systems (i.e., LTE: Long Term Evolution) that realize large-capacity and high-speed transmission is a transistor used for a power amplifier of a mobile communication base station. Such a transistor is required to possess a high-efficiency characteristic for reducing the size and the power consumption of the device.
One type of such a transistor is a field-effect transistor (FET) in which a nitride semiconductor such as GaN is used. GaN, which is a wide bandgap semiconductor, exhibits high saturated electron velocities and dielectric breakdown voltages. Further, since an AlGaN/GaN hetero junction structure can realize high sheet electron densities, GaN is expected to serve as a material for radio-frequency transistors that achieve high-voltage and high-output operations. The field-effect transistor employing GaN that can achieve high-voltage operations is capable of reducing the impedance transformation circuit loss, and operating highly efficiently.
Recently, what has principally been coming into practical use as the field-effect transistor for the mobile communication base station is a field-effect transistor that has a hetero junction structure, in which a GaN-based material is arranged on a SiC substrate having high thermal conductivity. A semi-insulating SiC substrate is extremely expensive currently and, therefore, cannot meet the needs for reducing costs. Accordingly, in order to achieve a good balance between costs and performances, a GaN-based field-effect transistor using a high-resistivity silicon substrate is also under development.
FIG. 8 shows a field-effect transistor disclosed in United States Patent Application Publication No. 2007/0272957. A field-effect transistor 110 shown in FIG. 8 includes a high-resistivity silicon substrate 120 whose resistivity is equal to or greater than 100 Ω·cm, an AlGaN layer 122, a GaN layer 112a, an Al0.26Ga0.74N layer 112b, a GaN layer 112c, a source electrode 114, a drain electrode 116, a gate electrode 118 that is provided between the source electrode 114 and the drain electrode 116, an SiN film 128, a source field plate 129 formed on the SiN film 128, and a protection film 124. The field-effect transistor in accordance with United States Patent Application Publication No. 2007/0272957 can reduce the RF loss involved with a substrate by using the high-resistivity silicon substrate, and can also reduce manufacturing costs by dispensing with an expensive semi-insulating SiC substrate.